This is a bit tricky. They both mean basically the same thing but are also quite different.
Only if is more defined and implies there is an ultimatum on something and it cannot happen unless there is something else involved for example "you can have ice cream, only if you finish dinner". This would mean it are not allowed to eat ice cream at all until dinner is finished.
As long as is a bit different and is more causal. "As long as you eat dinner you can have ice cream". They mean the same thing but one is more authoritative and final. A lot of the time if someone sad as long as they often won't follow through and are not that serious. Only if is very final and seems more urgent and important.

the two are very similar [without context] I would take them to mean:
only by - only this way can achieve the task
by only- just by doing this one thing you can complete the task...
in the first one there is one way to do it, in the second there may be many ways but this way there is only one step.
in the third statement it means that oxidization is the only thing that allows the oxygen to be consumed.. but that is a good example on how the context can change the meaning.

"Only" is used as an adjective or an adverb. When "only" is an adjective that modifies a noun, the noun must still use the normal rule for the articles "a," "an," and "the."
For example:
"You are the only one for me."
"You are the one for me."
As you can see, it is "the one," not "the only." You can place any adjective before "one" and still follow the same rule:
"You are the (adjective) one for me"
"You are the wrong one for me."
"You are the best one for me."
"You are the silliest one for me."
"You are the smartest, most talented, nicest, prettiest, most exiting, greatest one for me."
"You are the only good one for me."

The only thing I think is a bit unnatural, is that you said the word pretty fast, so that you almost couldn't hear the 'n' and the 'l'. It's fine though, but I recommend "pulling the word into the length" And also say it like "Ounly" :)